Membrane technology to reduce nitrogen in natural gas

ABSTRACT

An apparatus and process for providing purified natural gas wherein a natural gas feed mixture of hydrocarbons, nitrogen, and other permeable gases is provided to a semi-permeable membrane separator having a relatively higher selectivity for methane and other hydrocarbons and a relatively lower selectivity for nitrogen, to thereby provide a gaseous permeate product enriched in hydrocarbons and diminished in nitrogen.

This application claims priority from Provisional Application No.60/208,325, filed Jun. 1, 2000.

BACKGROUND OF THE INVENTION

Frequently natural gas contains excess nitrogen, making it commerciallyunusable. If wellhead natural gas has more than about 10.0 vol %nitrogen, then it may not have a minimum heating value specified by apipeline company. Until now, there has been no technology available toeconomically reduce nitrogen content in natural gas. As a result thereare many capped wells that remain unused.

Pipeline natural gas can contain up to about 10.0 vol % nitrogen if C₂+and higher hydrocarbons are added to increase heating value to a nominalcommercial standard heating of 1,000±20 Btu per standard cubic foot(SCF). The balance is predominantly methane, usually 80-95 vol %, andsmall amounts of carbon dioxide, usually 0.0-2.5 vol %.

In the past the nitrogen content posed no problem for the most commonapplication of natural gas space heating, if the heating value was1,000±20 Btu per SCF. There is a new use for natural gas as a fuel forfuel cells (e.g., ONSI Corporation's Phosphoric Acid Fuel Cell (PAFC))wherein a nitrogen content over 6.0 vol. % can severely reduce usefuloperating life.

The PAFC typically has two main operating sections:

-   -   1. A steam reformer where natural gas is partially oxidized by        steam over a catalyst to yield hydrogen and carbon dioxide.    -   2. A stack of bipolar fuel cells with concentrated phosphoric        acid electrolyte wherein hydrogen reacts electrochemically with        oxygen from air to produce electricity, heat and water.

The reformer catalyst can promote a side reaction between nitrogen, ifpresent in the natural gas, and hydrogen to form ammonia according tothe following chemical equation:

Although the conversion of nitrogen to ammonia is low, the upperallowable limit for useful stack life is 1.0 ppmv of ammonia in thereformed gas. The ammonia concentration appears to be directlyproportional to the amount of nitrogen in the natural gas. It has beenfound that a PAFC reformer creates about 1.0 ppmv ammonia if the naturalgas contains about 6.0 vol % nitrogen. The level of 1.0 ppmv ammonia isgenerally considered to be the maximum allowable ammonia content foroptimum cell stack life. At 1.0 ppmv ammonia, a PAFC cell stack wouldhave about six years useful operating life before it would have to berenewed or replaced.

In the cell stack, ammonia hydrolyzes to ammonium ion as ammoniumhydroxide, reacting with the phosphate ion in the aqueous phosphoricacid electrolyte. The ammonium ion neutralizes the phosphate ion in anelementary acid base reaction, forming the salt ammonium meta-phosphateaccording to the following chemical equation:

 4NH₄OH+4H₃PO₄→(NH₄)₄P₄O₁₂+8H₂O  (2)

Converting a portion of the electrolyte to the neutral salt degrades itshydrogen ion conductivity and eventually the rated electric power outputof the fuel cell. It has been found that a PAFC fueled by natural gaswith an average 6.0 vol. % nitrogen content and the resultant average1.0 ppmv ammonia created in the reformed gas has a useful operating lifeof about six years. It has furthermore been found that if the averagenitrogen content increases to 8.5 vol. %, there will be a proportionalincrease in ammonia concentration and an exponential reduction in usefuloperating life to about 1.5 years.

The ratio of fuel cell operating life, and therefore degradation rates,appears to vary directly with the fourth power of the ratio of nitrogenconcentration. The degradation ratio is the inverse of the life cyclesratio:(8.5 vol. %/6.0 vol. %)⁴=4.028; 1/4.028≈1.5 yrs/6.0 yr.  (3)(6.0 vol. %/8.5 vol. %)⁴=0.248; 1/0.248≈6.0 yrs/1.5 yr.  (4)

Elementary chemical reaction kinetics also supports this conclusion. Anirreversible reactant rate expression for the depletion (−r) of theammonium ion as ammonium hydroxide derived from the stoichiometricequation (2) is:−r=k×[C_(NH) ₄ _(OH)]⁴×[C_(H) ₃ _(PO) ₄ ]⁴  (5)where −r is moles per unit volume depleted per unit time, k is thetemperature dependent rate constant, and C is the reactant concentrationin moles per unit volume. The reaction rate is fourth order with respectto ammonium concentration. In other words, the rate of ammoniumconversion and hence the rate of electrolyte degradation varies as thefourth power of the ammonium concentration, which is directlyproportional to the nitrogen concentration.

SUMMARY OF THE INVENTION

To overcome the foregoing problems associated with high nitrogen, thepresent invention is now directed to an apparatus and a process by whicha gas separation membrane can separate a natural gas stream containingnitrogen into a permeate product stream reduced in nitrogen and anon-permeate recycle stream rich in nitrogen.

In a preferred embodiment, a silicone (poly-alkyl siloxane such aspoly-dimethyl siloxane) gas separation membrane has been found to beparticularly useful. It has been shown that this type of membraneselectively permeates hydrocarbons over nitrogen and in particular showsa moderate selectivity for methane and higher selectivity for C₂+ andhigher hydrocarbons over nitrogen. When pipeline natural gas containinghydrocarbons and nitrogen is fed to the membrane, a permeated streamenriched in hydrocarbons and diminished in nitrogen is produced. Thenon-permeate stream is reduced in hydrocarbons and enriched in nitrogen.

Furthermore it has been found that the non-permeate stream heating valuevaried inversely with the amount of C₂+ and higher hydrocarbons in thefeed, the permeate recovery rate, and, directly with the permeatepressure. The higher the fraction of feed components that are morepermeable than methane, the larger the heating value reduction in thenon-permeate. It has been found that if the C₂+ hydrocarbon content isless than 1.0% of the feed, then the non-permeate stream heating valuereduction is equal to or less than 1.0% compared to the feed. Increasingthe permeate pressure reduces permeate recovery and increases theheating value of the non-permeate stream.

If the non-permeate heating value is maintained at 99.0% or morecompared with that of the feed, then it may be returned to the pipelineat no cost to the PAFC user. The amount of heating value reductionvaries directly with the percentage of permeate recovery. The lower thepermeate recovery is, the lower the heating value reduction is, and viceversa. In this case the permeated product recovery rate is of noconsequence since the PAFC user only pays for the permeate productstream.

BRIEF DESCRIPTION OF THE DRAWING

A continuous range of permeate product (product) flows is controlled bythree input/output control loops in a process controller. The threeinput devices are: 1) Product pressure sensor (PPS) (control loop 2); 2)Product Flow Meter (PFM) (control loops 1 and 3); and 3) Recycle FlowMeter (RFM) (control loop 3).

DETAILED DESCRIPTION OF THE INVENTION

The percentage of feed that is recovered as reduced nitrogen permeateproduct is governed by:

-   (1) the higher hydrocarbons content in the feed;-   (2) the permeate pressure; and-   (3) the desired product recovery.

If C₂+ hydrocarbons and other components in the feed are equal to orless than 1.0 vol. %, then the non-permeate will have a heating valueequal to or greater than 99.0% of the feed. Non-permeate with 1.0% orless heating value reduction compared to the feed can be recycled backinto the pipeline for general use. A higher percentage of C₂+hydrocarbons in the feed increases the product recovery rate anddecreases the non-permeate stream heating value.

Increasing permeate pressure reduces permeate recovery and increasesnon-permeate heating value. For most natural gas compositions, theheating value of the non-permeate will be within 99.0% of the feed ifthe increase in permeate back pressure reduces the permeate recoveryrate to about 30.0% or less.

MEMBRANE PREPARATION

In a preferred embodiment a silicone monomer (e.g.,polydimethylsiloxane, hexamethyl disiloxane, etc.) is introduced into aplasma-generating vessel under vacuum, where it is polymerized andcross-linked in situ onto a micro-porous polymeric hollow fiber such aspolypropylene (e.g., Celgard X20-240 and Celgard X20-400 from HoechstCelanese and KPF190M and KPF205M-1 from Mitsubishi) or a polysulfone(e.g., Filtron MW Cut 10K Dalton from Pall). In the subject membrane asilicone coating of about 0.5 μm thick is plasma deposited on asupporting micro-porous polypropylene hollow fiber, of about 250 μmoutside diameter and about 200 μm inside diameter.

Natural gas, either at pipeline pressure, or compressed to a requiredmembrane operating pressure, is provided to a single membrane separatorunit or the first stage of a multiple membrane separator unit system.The permeate pressure is always lower than the feed pressure to ensure apartial pressure difference driving force across the membrane.

Normally a single membrane separator unit is sufficient if no more thana 30.0% nitrogen reduction is required. If more nitrogen reduction isnecessary, permeate can be re-compressed and provided to succeedingmembrane separator units until the desired nitrogen reduction isachieved. A succeeding membrane separator units' non-permeate isrecycled back to the feed of the first membrane separator, e.g., asdisclosed in U.S. Pat. No. 5,482,539.

The final membrane separator permeate is the eventual reduced nitrogennatural gas product. The first membrane separator's non-permeate can beeither sent to a special use consumer, if the heating value is less than99.0% of the feed, or recycled back to the gas utility distributionsystem if the heating value is 99.0% or more of the feed.

Tables 1 and 2 below present field test data that illustrates therelationship between the critical parameters of nitrogen and heatingvalue reduction, higher hydrocarbon content in the feed and permeateproduct recovery. Table 1 sets forth results with a natural gas feedhaving a C₂+ content of about 0.05 vol. %, and Table 2 for a feed with aC₂+ content of about 4.7 vol. %. The test membrane module was two feetlong by four inches in diameter having two cartridges each having about50 square feet of hollow fiber membrane area of the silicone typedescribed above.

In both Tables, Run 1 represents a test membrane operating at a higherrecovery than Run 2. Comparing Tables 1 and 2 it can be seen that aboutthe same permeate recovery rate gives a higher nitrogen reduction whenC₂+ is more than 1.0 vol. %.

When the C₂+ content is less than 0.1 vol. % as in Table 1, the heatingvalue reduction is less than 0.5% over a wider range of recovery ratescompared to Table 2. If the C₂+ content is about 5.0 vol. % as in Table2, then the heating value reduction decreases to about 1.0% or less asthe recovery approaches about 30.0% or less.

The drawing shows a nitrogen reduction membrane (NRM) apparatuscomprising a preferred embodiment of the invention. High nitrogencontent natural gas enters the NRM in feed header 1. The gas isdistributed into parallel membrane banks A, B, or C, depending whetheror not feed header valves FA, FB, or FC and permeate product headervalves PA, PB, or PC are open or closed. Non-permeate gas enriched innitrogen exits the NRM in non-permeate recycle header 2. Permeate gasdiminished in nitrogen exits the NRM in permeate product header 3.

TABLE 1 Natural Gas With Less Than 0.05% C₂+ Nitrogen Reduction FeedFlow, Non-Permeate Permeate Run scfh N₂, vol. % Flow, scfh N₂, vol. %Flow, scfh N₂, vol. % N₂, reduct. recovery 1 310 6.1 180 8.3 130 4.1 33%42% 2 470 5.9 300 7.5 170 4.1 30% 36% Heating Value (HV) Reduction FeedFlow, Non-Permeate Permeate Run scfh Btu/cf Flow, scfh btu/cf HV reduct.Flow, scfh btu/cf recovery 1 310 939 180 938 0.1% 130 961 42% 2 470 942300 939 0.3% 170 964 36%

TABLE 2 Natural Gas With More Than 4.7% C₂+ Nitrogen Reduction FeedFlow, Non-Permeate Permeate Run scfh N₂, vol. % Flow, scfh N₂, vol. %Flow, scfh N₂, vol. % N₂, reduct. recovery 1 877 4.3 482 5.6 395 2.6 38%45% 2 745 4.1 503 4.8 242 2.8 33% 32% Heating Value (HV) Reduction FeedFlow, Non-Permeate Permeate Run scfh Btu/cf Flow, scfh btu/cf HV reduct.Flow, scfh btu/cf recovery 1 877 1006 482 985 2.1% 395 1031 45% 2 7451007 503 997 1.0% 242 1028 32%

According to a preferred embodiment of the invention, the permeateproduct recovery rate is optimized by controlling one or both of thefollowing operating parameters:

-   -   1. Permeate product header pressure controlled by the number of        modules operating for a given product demand calculated in        control loop 1 using the product flow meter (PFM) as the input        process variable sensor. Recovery varies directly with the        consequent back pressure created by product control valve (PCV);        and    -   2. Feed flux (flow rate per unit area of membrane surface)        compared to a set point feed flux in control loop 3. Recycle        flow is restricted by the non-permeate recycle control valve        (RCV) % open to control feed flux to a set point calculated        using PFM+recycle flow meter (RFM) as the input process        variables. Recovery varies inversely with the feed flux.

Permeate product pressure is controlled by comparing the processvariable product pressure sensor (PPS) to a product pressure set pointin control loop 2. Permeate product pressure is maintained by thepermeate product control valve (PCV) % open calculated using PPS as theinput process variable.

Referring again to the drawing, the three output devices are: 1. On/offmembrane bank valve pairs; 2. Product Control Valve (PCV); and 3.Recycle control valve (RCV). Each membrane bank uses a pair of on/offfeed and product header control valves (FA/PA, FB/PB & FC/PC) thatdetermine product flow. The product control valve (PCV) controls theproduct flow and product pressure. The recycle control valve (RCV)controls the feed flux and product recovery.

While the invention has been described in detail and with reference tovarious embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A process for providing a purified natural gas permeate product froma membrane separator unit to a fuel cell, which permeate product has anitrogen content of 6% or less, comprising subjecting a feed gas mixtureof hydrocarbons, nitrogen, and other gases commonly associated withnatural gas to a semi-permeable membrane separator having a relativelyhigher selectivity for methane and other hydrocarbons and a relativelylower selectivity for nitrogen, to thereby provide a gaseous permeateproduct enriched in hydrocarbons and diminished in nitrogen, andproviding said gaseous permeate product to a fuel cell.
 2. A process asin claim 1, wherein there is provided a reduced nitrogen contentpermeate with a nitrogen reduction equal to or greater than 30.0% offeed nitrogen content.
 3. A process as in claim 1, wherein there isprovided an enriched nitrogen content non-permeate having a heatingvalue equal or greater than 99.0% of the heating value of the feed gas.4. A process as in claim 1, wherein there is recovered 70.0% or more ofthe feed gas mixture as non-permeate, and said non-permeate has aheating value of 99.0% or more than the heating value of the feed gasmixture.
 5. An apparatus wherein a natural gas feed mixture ofhydrocarbons, nitrogen and other permeable gases is provided to a feedheader connected to a system of membrane separators, which separatorsystem provides a permeate product enriched in hydrocarbons anddiminished in nitrogen to a product header directed to a fuel cell and anon-permeate recycle enriched in nitrogen and diminished in hydrocarbonsto a recycle header directed to a special use consumer or re-directedback to a gas utility distribution system.